Others have answered to explain that the brain can indeed do parallel processing, or how the structure of the brain and a computer differ. I'm gonna answer your "Why?":
Because your brain was "built" to do specific tasks, and not others.
Your question is like asking why a car cannot fly, if planes can fly. The answer is simple: Because cars were optimized to drive on a road.
Your brain evolved to be perfect at helping you hunt antelopes, interact with other humans in a complex society, and survive virus infections.
That was the answer to the "Why?" of your question. What follows here are some thoughts on the extent and the form of the differences between a computer and the brain.
Your brain – which, as others have pointed out, actually does massive parallel processing – did not evolve to do "scientific simulations, modeling, video/image processing etc.".
Oh, wait, doesn't the brain process images? Sure, it processes the input from the eyes, and the constant, video-like flow of visual information your eyes send to the brain is certainly larger than any digital image or video file (which, by the way, a computer may be able to perform interesting looking video effects on, but quite simply cannot even begin to understand). Oh, and doesn't the brain also do scientific simulations and modeling? Sure, or how do you think we have an idea of the world and solve problems? When I sit down and wonder "what if", that isn't any different from a "scientific simulation".
So, it seems to me that the brain does everything that you say it doesn't. It even scans a huge database (i.e. your whole memory) in milliseconds.
But let's get back to the part where I said that the brain even fights a viral infection of the body. This seems exceptionally important to me:
The brain is connected and regulates the whole of the body that it resides in.
A computer processor does not even note if a key on the keyboard no longer works, much less repair itself. In a computer, the processor is separate and barely connected to most of the other parts that make up the whole of the computer. Your brain is not a distinct part of your body. It is so intimately interwoven with it that it is difficult to tell where it begins and ends. From everywhere in your body nerve fibres lead to your brain, and your brain is in fact the collection of the endpoints of these nerve fibres. You may have heard of the second nerve center, the so-called "second brain" in your guts. You have more neurons in your intestines than in your spinal cord. In a computer, there are no other parts where there are processors or their axons, and the connections of the processor to the interfaces, the memory and a handful of other parts, are few.
And then neurons and their electricity are only one channel of information flow in the brain/body-whole. Information in the human body is stored and processed in the form of chemicals also. And in the form of growth and decay of cells. The form and shape of your body is in fact also memory, and this information is available to your brain.
In short, the brain and the body are an inseparable unit. As research into emotion shows,
"processing" of information does not take place only in the brain, but in the whole of the body,
all the time, and the amount of information that the sum of these – parallel! – processes make up is as yet unfathomed.
A computer is a badly designed box in which a processor is screwed that does mathematical calculations. A human being is an organism that survives and procreates. How anyone could think of comparing the two is a mystery to me.
Yes, the human brain cannot count from 1 to one billion as quickly as a computer can, but that seems like an irrelevant exercise to me anyway. If you think of other complex calculations like the weather, a human who is given the current weather data can predict the weather almost as accurately as any computer can. What does that tell us about the impressive calculation power of computers?
What? Google's computers can scan billions of web pages and create translations that are almost correct? Interesting. I can come up with translations that are correct. Well, sure, someday Google may have perfected that algorithm to no longer make mistakes, but that's a lot of effort, people, time, and money, put into doing something that half of humanity can already do, and the other half could learn if they could go to school.
What? Computer software can render a photorealistic landscape? Looks good. With all the time the modelling, texturing and light set up took, a good human painter can do the same in acrylic or oils. And the painting even looks less sterile and more alive :-)